Prostaglandin E(2) (PGE(2)) is a known inflammatory mediator of tendinitis, for which mechanical loading on tendons is believed to be one of the most prominent causation factors. Previous in vitro studies have shown that cyclic mechanical stretching of cells can cause changes in cell morphology and alteration of both DNA and protein syntheses. In our study, a novel system was used whereby tendon fibroblasts are cultured on microgrooved silicone surfaces and are subjected to cyclic uniaxial stretching along their long axes to mimic in vivo conditions. Using this unique model system, the cell shape and alignment can be controlled. Further, this study was designed to test the hypotheses that PGE(2) production increases in a stretching magnitude-dependent manner and that cyclooxygenase (COX) is responsible for the increased PGE(2) production in tendon fibroblasts. Human patellar tendon fibroblasts were cultured on the microgrooved silicone membranes and cyclically stretched at 4%, 8%, or 12% of nominal dish length for 24 hr. PGE(2) production was found to be increased 1.7-fold at 8% cyclic stretching and 2.2-fold at 12% cyclic stretching compared with nonstretched controls. In addition, human tendon fibroblasts had increased expression of both COX-1 and COX-2 for all three applied stretching magnitudes, with the exception of COX-1 at 4% cyclic stretching. Also, cellular PGE(2) production, after 8% cyclic stretching, was significantly decreased with the addition of indomethacin (25 microM), a COX competitive inhibitor, compared with stretched cells without indomethacin treatment. These findings suggest that the increase in PGE(2) production by the human tendon fibroblasts is stretching magnitude-dependent, and that the increase in COX expression contributes to the increased production of PGE(2) after cyclic stretching. As PGE(2) is a known inflammatory mediator of tendinitis, the contribution of COX-1 and COX-2 to PGE(2) production and their roles in tendon inflammation are clearly indicated.